1. Field of the Invention
The present invention relates to a self-holding type solenoid-operated valve which can increase the attracting force and the holding force for a movable iron core.
2. Description of the Related Art
A solenoid-operated valve has been used for controlling the flow direction of the compressed air by supplying the compressed air to an actuator or discharging the compressed air to the atmospheric air.
A conventional latch type solenoid-operated valve of this type is shown in FIG. 7 (for example, see Japanese Laid-Open Patent Publication No. 63-297883).
The solenoid-operated valve comprises an electromagnetic coil 3 including a valve-switching electromagnetic coil 2a and a valve-restoring electromagnetic coil 2b which are disposed coaxially with a bobbin 1. A permanent magnet 4 is axially disposed on an end of the electromagnetic coil 3. A fixed iron core 5 is disposed coaxially with the permanent magnet 4 in the bobbin 1.
A main valve 7 having a valve stem 6 is disposed on another end of the electromagnetic coil 3 in the direction opposite to the permanent magnet 4. An end of a plunger 8 of a movable iron core is secured to the valve stem 6. Another end of the plunger 8 is inserted displaceably into the center of the electromagnetic coil 3.
The valve stem 6 is urged in the direction opposite to the plunger 8 by a spring 9 which serves as an urging mechanism. The valve stem 6 indirectly urges the plunger 8 toward the direction opposite to the fixed iron core 5.
The operating principle of the conventional latch type solenoid-operated valve thus constructed will now be briefly explained.
FIG. 7 shows a non-excited state in which no current flows through the valve-switching electromagnetic coil 2a and the valve-restoring electromagnetic coil 2b respectively. FIG. 7 also shows an initial position in which the main valve 7 is closed. The plunger 8 is being in the initial position because the spring force (fs) of the spring 9 is larger than the force (F) of the permanent magnet 4.
Thereafter, as shown in FIG. 8, when the electric power is applied to the valve-switching electromagnetic coil 2a, the attracting force (fa) of the solenoid is generated by the magnetic excitation of the valve-switching electromagnetic coil 2a. When the force (fa+F) generated by adding the attracting force (fa) to the force (F) of the permanent magnet 4 becomes larger than the spring force (fs) of the spring 9, the plunger 8 is attracted toward the fixed iron core 5. Accordingly, the main valve 7 is opened.
The electric power ceases to be applied to the valve-switching electromagnetic coil 2a with the main valve 7 being opened. Then, the holding force of the permanent magnet 4 is larger than the spring force (fs) of the spring 9. Therefore, the plunger 8 is being attracted toward the fixed iron core 5 even if the electric power ceases to be applied thereto.
If the electric power is applied to the valve-restoring electromagnetic coil 2b, the attracting force (fb) of the solenoid is generated under the magnetic excitation of the valve-restoring electromagnetic coil 2b. The attracting force (fb) of the solenoid counteracts the holding force of the permanent magnet 4 (see FIG. 9). Therefore, the plunger 8 is restored to the initial position shown in FIG. 7 by the spring force (fs) of the spring 9. Accordingly, the main valve 7 is closed.
However, the conventional latch type solenoid-operated valve requires both of the valve-switching electromagnetic coil 2a for driving the plunger 8 and the valve-restoring electromagnetic coil 2b for releasing the self-holding force of the permanent magnet 4 to restore the plunger 8 to the original position. Therefore, the space occupied by the coil is increased. The entire apparatus of the solenoid-operated valve becomes inevitably large and the production cost becomes expensive.
The permanent magnet 4 is disposed at the axial end of the electromagnetic coil 3 opposite to the plunger 8. When the large holding force is required to retain the plunger 8, the strong magnetic force is required with the large permanent magnet 4. It is also necessary to use the valve-restoring electromagnetic coil 2b for generating the large inverted magnetic force to release the holding force. Accordingly, the consumption of the electric power is increased.
It is a general object of the present invention to provide a self-holding type solenoid-operated valve which can minimize the entire apparatus and can realize low electric power consumption.